Vibration isolation systems are known and are required in several technique areas, as for instance in the area of semiconductor industry. For instance document DE 69817750 T2 shows a vibration isolation system which is particularly provided for a vibration isolated positioning of a lithography-device. It is shown a load which is positioned on air bearings to isolate the load from vibrations and/or oscillations. Typically the load comprises a table and components which are positioned on the table. One example of such a component represents a production line or parts of it. The table is also known as base element or support body or main plate.
A passive vibration isolation system is characterized by a “simple” bearing having a mechanical stiffness as low as possible. The transmission of external vibrations to the isolated load is reduced. An air bearing and a polymer-spring-element for bearing represent two examples for a passive vibration isolation system.
The passive vibration isolation systems are characterized by a kind of oscillation damping (or damping attenuation) or by a kind of “isolated” bearing of the load. In contrast to this an active vibration isolation is characterized by an active vibration compensation. A motion which is induced by an oscillation and/or vibration is compensated by a corresponding counter motion. For instance an acceleration vector which is induced by an oscillation of the load is counteracted by an acceleration vector having the same absolute value but acting in an opposite direction (or having an opposite algebraic sign respectively). The resulting entire or total acceleration of the load is equal to zero. The load rests or remains in a desired position.
Therefore, active vibration or oscillation isolation systems comprise, optionally together with a bearing of mechanical stiffness as low as possible, additionally a regulating (or controlling) system. The regulating system comprises a regulating (or controlling) device, sensors as well as actuators. The actuators specifically counteract vibrations entering the system. The sensors detect the motions of the carried load. The regulating device generates compensation signals. By means of the compensation signals the actors are controlled and corresponding compensation motions are generated. It is possible to apply digital or analog regulating techniques or both techniques together as well, the so called hybrid regulating (or controlling) technique.
In general four air bearings per system and in particular per load are applied. The air bearings are called as well as pneumatic isolators or vibration isolators. Each of the isolators “alone” is used for height adjustment only. The system is redundant. Therefore, only three of the four air bearings are equipped with particularly controllable (or adjustable) valves. Since one plane, in this case the horizontal plane, is defined unambiguously by three support points. The forth isolator is supplied passively by one valve selected from the three further valve (a so called “master-slave-configuration”). Accordingly, in each case only three of the four isolators are influenced by means of a controlling. The forth isolator remains passive.
The requirements for vibration isolation systems are increasing, in particular in semiconductor industry. For instance moving or travelling or movable mounted tables on the support body, so-called “stages”, which are intended for the transportation of wafers are in operation. Due to their motion upon the support body, these stages induce vibrations in the system itself, so-called “intrinsic” vibrations. Therefore, it is necessary to compensate efficiently the vibrations induced by the motion of a stage. Since the stages own a quite high weight, increased demands are required to a system for vibration isolation or vibration compensation. In particular the “permanent” providing of the required force by the used actuators proves to be problematic.